Liposomes are microscopic vesicles comprised of single or multiple phospholipid bilayers which can entrap hydrophilic compounds within their aqueous cores. Liposomes have been formed in sizes as small as tens of Angstroms to as large as a few microns. Most liposomes are non-toxic, non-antigenic and biodegradable in character since they have the molecular characteristics of mammalian membranes.
Liposomes are used as carriers for drugs. Liposomes can be made with different features which can enhance a drug's efficacy; reduce a drug's toxicity; and prolong the drug's therapeutic effect.
Liposomes with multiple bilayers are known as multilamellar vesicles (MLVs). MLVs are excellent candidates for time release drugs because the fluids entrapped between layers are only released as each membrane degrades. Liposomes with a single bilayer are known as unilamellar vesicles (UV). UVs may be made extremely small (SUVs) or large (LUVs).
Liposomes are prepared in the laboratory by sonication, detergent dialysis, ethanol injection, French press extrusion, ether infusion, and reverse phase evaporation. These methods often leave residuals such as detergents or organics with the final liposome. From a production standpoint, it is clearly preferable to utilize procedures which do not use organic solvents since these materials must be subsequently removed.
Some of the methods impose harsh or extreme conditions which can result in the denaturation of the phospholipid raw material and encapsulated drugs. These methods are not readily scalable for mass production of large volumes of liposomes.
Several methods exist for producing MLVs, LUVs and SUVs without the use of organic solvents. MLVs, free of organic solvents, are usually prepared by agitating lipids in the presence of water. The MLVs are then subjected to several cycles of freeze-thawing in order to increase the trapping efficiencies for water soluble drugs. MLVs are also used as the starting materials for LUV and SUV production.
One approach of creating LUVs, free of organic solvents, involves the high pressure extrusion of MLVs through polycarbonate filters of controlled pore size. SUVs can be produced from MLVs by sonication, French press or high pressure homogenization techniques. High pressure homogenization has certain limitations. High pressure homogenization is useful only for the formation of SUVs. In addition, high pressure homogenization may create excessively high temperatures. Extremely high pressures are associated with equipment failures. High pressure homogenization does not insure end-product sterility. High pressure homogenization is associated with poor operability because of valve plugging and poor solution recycling.
The use of liposomes for the delivery and controlled release of therapeutic drugs requires relatively large supplies of liposomes suitable for in vivo use. Ostro, M. J. and Cullis, P. R., "Use of Liposomes as Injectable Drug Delivery Systems", American Journal of Hospital Pharmacy, 46:1576-1587 (1989). Present laboratory scale methods lack reproducibility, in terms of quantity and quality of encapsulated drug, lipid content and integrity, and liposome size distribution and captured volume. The multidimensional characteristics of the drug and the liposome, as well as potential raw material variability, influence reproducibility.
Present liposome products are not stable. It is desirable to have final formulations which are stable for six months to two years at room temperature or refrigeration temperature. Stability requirements have been relaxed by techniques for dehydrating liposomes. Dehydrated liposomes can be distributed to hospitals free of drugs and mixed with the drug immediately prior to use by a hospital pharmacist. However, compounding of the liposome containing drug by a pharmacist increases the cost of the therapy and adds further potential for compounding errors.
Present liposome products are difficult to sterilize. Sterility is currently accomplished by independently sterilizing the component parts--lipid, buffer, drug and water--by autoclave or filtration and then mixing in a sterile environment. This sterilization process is difficult, time consuming and expensive since the product must be demonstratively sterile after several processing steps.
Heat sterilization of the finished product is not possible since heating liposomes does irreparable damage to liposomes. Filtration through 0.22 micron filters may also alter the features of multilayered liposomes. Gamma ray treatment, not commonly used in the pharmaceutical industry, may disrupt liposome membranes. Picosecond laser sterilization is still experimental and has not yet been applied to the sterilization of any commercial pharmaceutical.
There exists a need for large scale cost effective liposome manufacturing processes which can meet the growing market demand for liposomal drug delivery and controlled release vehicles. The process and equipment should recycle unentrapped drugs, lipids and solvents. The process and equipment should produce uniform liposome products. The ability to operate continuously is an added benefit to the process.